Question

Bronsted-Lowry acid among the following is :

• A. $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$
• B. $[FeC{l}_4{]}^{-}$
• C. $\left[Fe\right(H_{2}O{)}_6{]}^{3+}$
• D. $\left[Zn\right(OH{)}_4{]}^{2-}$

Answer 1

Answer: (C)

$\left[Fe\right(H_{2}O{)}_6{]}^{3+}$

Option (C) is correct. $\left[Fe\right(H_{2}O{)}_6{]}^{3+}$ is an example of Bronsted-Lowry acid.

When a coordinate covalent bond is formed between a metal cation and a water molecule, the positive charge of the metal ion and its small size means that the electron of the $H_{2}O\to M^{n+}$ bond are strongly attracted to the metal. As a result, the $O-H$ bonds of the bound water molecules are polarized. The net effect is that a hydrogen atom of the coordinated water molecule is removed as $H^{+}$ more readily than in an uncoordinated water molecule. Thus, a hydrated metal cation functions as a Bronsted acid or proton donor.

$\left[Fe\right(H_{2}O{)}_6{]}^{3+}+H_{2}O\rightleftharpoons \left[Fe\right(H_{2}O{)}_5\left(OH\right){]}^{3+}+H_3{O}^{+}$